The present invention relates to a gas liquefying system for gases having extremely low boiling temperature such as nitrogen, oxygen and so forth separated from air by an air separator or the like apparatus and, more particularly, to a gas liquefying system which liquefies gases of the kind mentioned above by using, as a cold heat source, a dual stage expansion turbine having a high-pressure stage turbine and a low-pressure stage turbine.
Japanese Patent Publication No. 40547/1974, for example, discloses a gas liquefying system which can liquefy gases having extremely low boiling temperatures such as oxygen, nitrogen or the like at a high efficiency, by using a dual stage expansion turbine having a high-pressure expansion turbine and low-pressure expansion turbine.
In general, when the liquefied gas as the product is supplied to a storage tank or the like, it is necessary to reduce the pressure of the liquefied gas to a considerably low level. Therefore, if the liquefied gas has not been super-cooled sufficiently, a part of the liquefied gas may evaporate. Namely, in order to prevent the flushing loss, it is essential to super-cool the product liquefied gas to a temperature which is equal to the saturation temperature after the pressure reduction. To this end, the gas temperature at the outlet from the low-pressure expansion turbine has to be decreased to a level not higher than the above-mentioned saturation temperature. An extreme low temperature at the outlet from the low-pressure expansion turbine, however, causes a part of the gas expanded through the turbine to be liquefied to generate mist. In general, the expansion turbine operates at a high speed of several tens of thousand revolution per minute. The liquid mist suspended by the gas, therefore, impinges upon the turbine blade to cause a rapid wear and unbalance of mass of the rotary part of the turbine, resulting in a breakdown of the turbine in the worst case. In order to obviate this problem, Japanese Patent Publication No. 40547/1974 proposes a method in which a part of the gas at the inlet to the high-pressure expansion turbine is introduced directly to the inlet side of the low-pressure expansion turbine thereby to control the gas temperature at the outlet from the low-pressure expansion turbine.
On the other hand, the temperature and the pressure of the gas at the inlet to the expansion turbine are preferably high, in order to attain a large theoretical adiabatic heat drop, from the view point of theory of thermodynamics. It is, therefore, preferred to elevate the gas temperature at the turbine inlet within the range allowed by the heat exchanger, for attaining a high efficiency of the gas liquefying system.
A conventional gas liquefying system employing a combination of high- and low-pressure expansion turbine will be explained hereinunder with reference to FIG. 1.
Referring to FIG. 1, the conventional gas liquefying system has a circulation type compressor 1 for compressing nitrogen gas, a pre-cooler 2, a cooler 3 making use of Freon or the like refrigerant, a heat exchanger 4, a liquefier 5, a high-pressure expansion turbine 6, a low-pressure expansion turbine 7, a liquefied gas discharge valve 8, a pipe 9 through which a part of the nitrogen gas cooled in the heat exchanger is introduced to the high-pressure expansion turbine for generating the cold heat, a pipe 10 through which the remainder of the gas is introduced as a liquefying gas to the liquefier 5, and a pipe 11 connecting the outlet of the high-pressure expansion turbine 6 to the low-pressure expansion turbine 7 past the liquefier 5.
In operation, after being compressed to a pressure of about 35 Kg/cm.sup.2 G by the circulation type compressor 1, the nitrogen gas is cooled through the pre-cooler 2 and the cooler 3, and is further cooled through the heat exchanger 4 by the returning gaseous nitrogen down to a low temperature of about -100.degree. C. The nitrogen gas then shunts into two parts. A first part of this compressed nitrogen gas is introduced into the high-pressure expansion turbine 6 through the pipe 9 and is expanded to a mean pressure of about 5 Kg/cm.sup.2 to generate a cold heat of about -160.degree. C. This cold nitrogen gas is introduced to the liquefier 5 in which the temperature of the gas is raised to about -150.degree. C. The gas is then introduced to the low-pressure expansion turbine 7 and expanded to a pressure of about 0.3 Kg/cm.sup.2 G to generate a cold heat of about -190.degree. C. The nitrogen gas of low temperature and pressure from the low-pressure turbine 7 is introduced to the liquefier 5. Meanwhile, the other part of the high-pressure liquefaction gas, shunted at the outlet of the heat exchanger 4, is introduced to the liquefier 5 and is liquefied and super-cooled by the cold heat of the low pressure and temperature nitrogen gas coming from the low-pressure expansion turbine 7. The nitrogen gas of low temperature and pressure then cools the high-pressure nitrogen gas flowing through the heat exchanger 4 and, after recovering the temperature through the heat exchange with the high-pressure gas, returns to the circulation type compressor 1 through the pre-cooler 2. On the other hand, the nitrogen gas of high pressure now liquefied in the liquefier 5 is super-cooled to the saturation temperature of the product gas while it flows through the downstream part of the liquefier 5 and is picked up as the product liquid nitrogen through the liquefied gas discharge valve 8. The product gas is then stored in a storage tank or used as the cold heat source for a rectification tower such as an air separator. Hitherto, the temperature regulation of the gas at the outlet from the low-pressure expansion turbine 7 is conducted by means of a temperature regulator 13 which operates the liquefied gas discharge valve 8 in response to a signal from a temperature sensor adapted to sense the gas temperature at the outlet side of the low-pressure expansion turbine 7. When the system operates with reduced quantity of the nitrogen gas or when the temperature adjustment by the liquefied gas is not available fully, the temperature regulation is conducted with the assist by the method proposed in Japanese Patent Publication No. 40547 mentioned before.
In this conventional method of regulating the temperature of the liquefied gas, the gas temperature at the inlet to the low-pressure expansion turbine, recovered by the liquefier 5, is changed depending on the flow-rate of the high-pressure gas to be liquefied which in this case serves as the hot heat exchanging medium. Partly because the heat transfer area of the liquefier 5 is unchangeable and partly because the temperature at the output from the low-pressure expansion turbine 7 is controlled preferentially, the temperature of the liquid nitrogen taken out of the liquefier 5 as the product is largely changed by the fluctuation of the flow rate of the gas to be liquefied. At the same time, the temperature at the inlet to the high-pressure expansion turbine 6 is affected.
The rise in the temperature of the liquefied nitrogen causes an increase in the flushing loss, to waste the nitrogen unnecessarily, resulting in a reduction of the efficiency. The method disclosed in Japanese Patent Publication No. 40547/1974, consisting in directly supplying a part of the gas from the inlet side of the high-pressure expansion turbine 6, is effective from the view point of protection of the low-pressure expansion turbine 7, but causes a loss of energy to decrease the overall efficiency of the liquefying system undesirably.